1. Field of the Invention
The present invention relates to a pulse wave period measuring apparatus, a pulse wave period measuring method, a heartbeat period measuring method, and a recording medium.
2. Background Art
A photoelectric pulse wave sensor is configured to measure a pulse wave by light. The sensor irradiates a light beam into a living body, and measures a reflected or transmitted light beam. When the light beam is irradiated into the living body, the light beam is absorbed by hemoglobin in the living body. Therefore, the intensity of the reflected or transmitted light beam varies according to the amount of blood. This implies that the intensity of the reflected or transmitted light beam varies synchronized with a heartbeat. Therefore, the photoelectric pulse wave sensor can measure a pulse wave of a subject by measuring the reflected or transmitted light beam. The sensor is used, for example, for monitoring a heart rate during exercise or the like.
The pulse wave is relatively easy to be measured, in comparison with other biological indicators such as an electro-cardiograph. In general, the pulse wave sensor can be easily attached, and is suitable for use in daily life. On the other hand, the pulse wave often fluctuates greatly in amplitude and baseline, due to narrowing and ballooning of a blood vessel or a breathing disturbance. In addition, it has been known that a subject's motion causes a change in contact state between the sensor and the subject's body and a change of the internal blood flow in the subject's body, which greatly disturbs the waveform of the pulse wave. It is desirable to take account of the above-mentioned merits and demerits when the pulse wave is measured. It should be noted that a heart rate can be calculated by detecting a pulse wave period. The accuracy of detecting the pulse wave period is greatly affected by a distortion or deformation of the pulse wave. Therefore, it is required to provide methods of stably measuring the pulse wave and of determining whether or not the pulse wave is stably measured.
The pulse wave sensor is applicable, for example, to a device for easily monitoring, at home, a sleeping condition or a health condition during sleep. In recent years, significant efforts have been made to develop such a device. JP-A 2002-291710 (KOKAI) discloses a method which calculates a pulse beat period from the pulse wave measured during sleep, performs a frequency analysis on the pulse beat period, and determines the sleeping condition based on the analyzed result of the frequency analysis. More specifically, the method converts a series of pulse beat period data calculated from the pulse wave, into a frequency spectrum distribution, extracts a predetermined frequency band component from the frequency spectrum distribution, and determines the sleeping condition, based on the value obtained by integrating the frequency component.
However, the method disclosed in the JP-A 2002-291710 (KOKAI) has a problem that if noise is contained in the series of pulse beat period data, the noise has a significant effect on the frequency spectrum. Therefore, it is necessary to remove the noise contained in the pulse beat period, due to a distortion or deformation of the pulse wave.
Further, JP-A 2004-89314 (KOKAI) and JP-A 2001-198094 (KOKAI) disclose methods for detecting a distortion of a pulse waveform. The method disclosed in JP-A 2004-89314 (KOKAI) accumulates a reference waveform in advance, calculates a correlation value between the reference waveform and an observed waveform, and determines whether the observed waveform is normal or abnormal, based on the correlation value. The method disclosed in JP-A 2001-198094 (KOKAI) measures pulse wave periods for plural pulse beats, and determines whether an observed waveform is normal or abnormal, based on the variation of the pulse wave periods.
However, according to the method disclosed in JP-A 2004-89314 (KOKAI), when measuring each waveform, it is necessary to extract a waveform of a pulse beat and calculate a correlation between waveforms, so that the processing load is quite heavy. In particular, when processing is performed in the sensor, a high performance computer is required for the processing, and thus a larger computer is required. In addition, since increasing power consumption is expected for the processing, a larger battery is required. In this way, a larger sensor is required when the processing is performed in the sensor. Further, according to the method disclosed in JP-A 2001-198094 (KOKAI), since pulse wave periods for plural pulse beats are required, it takes too much time for the determination, and it is difficult to determine whether the waveform is normal or abnormal for each waveform. In fact, when a pulse wave is measured, it is often the case that only one waveform is abnormal due to a momentary motion or noise. However, the method cannot adequately deal with such a case. The methods disclosed in JP-A 2004-89314 (KOKAI) and JP-A 2001-198094 (KOKAI) have such problems.